This invention relates to spark timing control in an internal combustion engine, and particularly to a system for adaptively controlling engine spark timing to achieve substantially peak thermal efficiency.
Typical spark timing control systems for internal combustion engines sense a set of spark timing dependent variables, such as engine coolant temperature, engine speed and engine load, and adjust the ignition timing to a prescribed calibration for a baseline engine. Generally in these systems, the spark timing angle is retrieved from stored tables addressed by the spark timing dependent variables with additional retard, in some cases, in response to knock, manifold absolute pressure or a throttle movement detector. A problem with this approach is that, because of manufacturing differences and aging, the engine being controlled is not necessarily the same as the baseline engine used for reference.
One approach to the solution of this problem requires extensive effort in the design of the engine to ensure both that all combustion chambers behave the same and that components will not significantly alter characteristics with aging. Even with such design and type quality control in the manufacturing process, the required uniformity and stability over time of such engines cannot always be achieved.
Closed loop control of spark timing has been suggested in the prior art in various forms. Location of peak pressure spark timing control systems have been proposed in which spark timing is controlled to maintain a predetermined location of peak combustion pressure. This timing of peak combustion pressure has been found to produce MBT operation for many engine operating conditions. The location of peak pressure may be sensed using head bolts with an embedded piezoelectric material which responds to the stress created in reaction to the pressure on the engine cylinder head. However, this approach has difficulties when the combustion charge is highly dilute or the engine is under light load.
Some systems have been suggested which adjust ignition timing to control the maximum absolute combustion pressure with respect to some predetermined reference level. There is, in addition, a suggestion in the U.S. Pat. No. Greeves 4,449,501 issued May 22, 1984, that ignition timing may be controlled to maintain the ratio of maximum combustion chamber pressure to maximum motored pressure in accordance with a stored table addressed by engine speed and load factors. These approaches, however, involve stored references determined for a baseline engine which may not be the desired reference for the particular engine at any given time.
Another known system provides for the closed loop control of ignition timing by controlling the middle of the combustion event to a predetermined angle after top dead center. However, in this system, certain assumptions were made in determining the middle of the combustion event or burn center. These assumptions include (1) the duration of combustion is 120 degrees and (2) the mass burn rate is assumed to increase linearly with crankshaft rotational angle up to the 50% mass burned point and then to decrease linearly to the end of combustion. However, it has been observed that the duration of combustion may vary substantially from 120 degrees. For example, the duration may vary between 40 and 150 degrees. Further, it has been observed that the burn rate varies non-linearly. Accordingly, an error in the determination of the 50% mass burned point may result.